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Hearing Threshold Measurements of Infrasound Combined with Audio Frequency Sound 12th ICBEN Congress on Noise as a Public Health Problem Hearing threshold measurements of infrasound combined with audio frequency sound Elisa Burke1, Johannes Hensel1, Thomas Fedtke1 1 Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany Corresponding author's e-mail address: [email protected] ABSTRACT Within the framework of the European project EMPIR 15HLT03 "Ears II" this study aims at a better understanding of the human response to infrasound. The purpose of this study is to examine which role the combination of infrasound (< 20 Hz) and sound in the audio frequency range (between 20 Hz and 20 kHz) plays for the perception of infrasound. One hypothesis to be validated is that the interaction between infrasound and audio-frequency sound may explain the perceptibility of infrasound. Another aim is to investigate whether the presence of infrasound influences the hearing threshold of audio frequency sound. In order to test these hypotheses detection threshold measurements were performed separately for infrasound and audio-frequency sound stimuli. Then thresholds were measured for infrasound stimuli in the presence of audio-frequency sound and for audio-frequency sound stimuli in the presence of infrasound. The measurement setup consisted of an infrasound source and an audio-frequency sound source, each coupled by a sound tube to the same eartip that was used for monaural presentation of the acoustic stimuli. 1. INTRODUCTION An increasing number of individuals are being exposed to infrasound. It is well known that certain individuals may be particularly sensitive and that their quality of life is considerably degraded by a range of symptoms (insomnia, concentration disorders, restlessness, migraine), e.g. [1]. Unfortunately, even technology indispensable for a sustainable development in the European Union (like renewable energy technology and road traffic) also produces infrasound noise. It is, therefore, an important overall need with respect to many aspects of quality of life to gather more basic knowledge about infrasound perception and impact mechanisms. Infrasound detection thresholds, loudness estimates and other psychoacoustical characteristics are reported from a number of studies, e.g. [1, 2, 3]. 1 It is, however, still not clear, how human beings process infrasound. Our study aims at contributing to knowledge about the perception mechanisms for infrasound and low-frequency sound by investigating the interaction between infrasound and audio-frequency sound with respect to the detection thresholds. Moreover, we think it is worth investigating whether the presence of infrasound influences the hearing threshold of audio-frequency sound. This study confines the perception of infrasound to “hearing by means of the human auditory system”. All other ways of sensation, e.g. somatosensory mechanisms, are intentionally ruled out for our investigation. Therefore we used insert earphone sound sources, both for the infrasound and the audio-frequency stimuli. The results presented in this conference paper are outcome of a pilot study for further research within the framework of the project. 2. MATERIALS AND METHODS 2.1. Measurement setup Figure 1: Schematic view of setup for the detection threshold measurement of infrasound and audio frequency sound stimuli using the insert earphone sound source system. In order to obtain a controlled auditory stimulation with infrasound combined with audio frequency sound we used a specially developed insert earphone sound source system (see Figure 1). The sound source system consisted of an infrasound source and an audio- frequency sound source, both realized as damped wooden boxes into which loudspeakers were mounted hermetically. Their sound was transferred to the audiometric eartip (E-A-RTone/ E-A-RLink, Standard Insert Foam Eartips) by means of connecting tubes. The 2 eartip was inserted into the test subject’s right ear for monaural presentation of the acoustic stimuli. The contralateral ear was occluded with an ear plug. The test subject as well as the sound sources and the sound tubes were located in an anechoic room with sufficiently low background noise levels. A computer display and a keyboard, which were necessary for the experimental procedure (see section 2. 5., Psychoacoustic measurement procedure), were placed in front of the subject. The computer controlling the experiments was located outside the anechoic room. The infrasound and the audio-frequency waveforms were calculated with MATLAB at 96 kHz sample rate. An external sound card (RME Fireface UC; infrasound and audio-frequency components in separate channels) generated analogue output signals that were fed to three separate amplifiers (type Beak BAA 120 for the compensation and the infrasound source, type Tira BAA 120 for the audio sound source). The infrasound source contained a 13“ electrodynamic loudspeaker (Beyma 15P80/Nd) generating the infrasonic pure-tone stimuli with sufficiently high sound pressure levels at low harmonic distortion [4]. The upper 8“ electrodynamic loudspeaker (Beyma 8P300Fe/N) inside the combined source (hereafter referred to as audio sound source) generated the stimuli in the audio frequency range up to 4 kHz. A major difficulty associated with the combination of infrasound and audio-frequency sound is that infrasound which is transferred by the sound tube to the audio sound source may cause a technical modulation by displacing the audio sound source loudspeaker's membrane. In order to compensate this modulation, down to a sufficiently (i.e., imperceptible) low level, an additional 8“ loudspeaker (Beyma 8P300Fe/N) (hereafter referred to as compensation source) was mounted in the box, below the audio sound source. The compensation source emitted infrasound with the same frequency as the infrasound source but with a frequency-dependent time shift adjusted with a time delay unit (part of Behringer Ultradrive Pro DCX 2496). Second-order low-pass filters (fc = 25 Hz) were inserted between the amplifiers and the loudspeakers for the infrasound source and the compensation source, and a second-order high-pass filter (fc = 460 Hz) as well as a 30 dB attenuator were inserted into the signal path of the audio sound source for suppressing the amplifiers' 50 Hz hum and its harmonics sufficiently to be well below the normal hearing threshold level. 2.2. Stimuli In the low-frequency range we used pure-tone stimuli at 12 Hz and 20 Hz. Band limited pink-noise stimuli were applied in the audio-frequency range: a one-third octave pink noise and a four octave wide pink noise, both centred at 1 kHz. The pink-noise signals were digitally pre-shaped by multiplying them with the inverted frequency response of the audio sound source in order to get a flat acoustical spectrum of the stimuli. All stimuli were windowed with a cos² function of a well-defined duration. Three oscillations for each cos² ramp were chosen for the 12 Hz stimuli, resulting in 0.25 s duration of each cos² ramp. The duration of the cos² ramps applied to the 20 Hz pure tone and to the pink-noise stimuli was 0.2 s. The sound pressure levels of the low-frequency stimuli were calibrated with a ½“ low- frequency pressure-field microphone (Brüel & Kjær, type 4193 + UC0211) that was placed in a cavity having an equivalent ear canal volume of 1.3 cm³, whereas the audio-frequency pink- 3 noise stimuli were calibrated with an occluded-ear simulator (Brüel & Kjær, type 4157 + ear canal extension DB 2012). The sound tubes from the sound sources were coupled to the cavity and the ear canal extension, respectively, by means of the eartip. 2.3. Experimental design The test subject received written and oral instructions prior to the beginning of the first experiment. The threshold measurements performed in this study were separated in three experiments (see Table 1). Each experiment was divided in several experimental runs. One experimental run comprised one threshold measurement of the target stimulus in silence or in presence of a background stimulus at a specific sound pressure level. Table 1: Overview of the experimental design of the detection threshold measurements. Target stimulus Background stimulus Sound pressure level of background stimulus / dB SL Experiment 1 Pink noise 250 - 4000 Hz - - Pink noise 890 - 1121 Hz - - 20 Hz* - - 12 Hz - - Experiment 2 12 Hz Pink noise 250 - 4000 Hz 0, +35, +65 12 Hz Pink noise 890 - 1121 Hz 0, +35, +65 Experiment 3 Pink noise 250 - 4000 Hz 12 Hz -10, 0, +10 Pink noise 890 - 1121 Hz 12 Hz -10, 0, +10 *The experimental run at 20 Hz was only intended to allow the subjects to familiarize themselves with the unusual perception of low-frequency sound. First (see Table 1, Experiment 1) detection threshold measurements were performed for infrasound and audio-frequency stimuli in silence. The experiment began with detection threshold measurements of the two bandlimited pink-noise audio-frequency signals, followed by a shortened detection threshold measurement for 20 Hz, which was only intended to allow the subjects to familiarize themselves with the unusual perception of low-frequency sound. Therefore it was not included in the measurement evaluation. Finally, the detection threshold of the 12 Hz infrasonic stimulus was measured. The two following experiments comprised detection threshold measurements of infrasound stimuli in the presence of audio-frequency sound (Experiment 2) and detection threshold measurements of audio-frequency stimuli in the presence of infrasound (Experiment 3). Both experiments were separated
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